For an ideal liquid solution at modest pressure, a is just the mole fraction x. And for an ideal gas, a is just its partial pressure in bar:

$$ K=\left.\frac px\,\right|_\text{equil}$$

One equilibrium condition is $x=1\ $ and $p=p^*,\ $ so...

$$ K=p^* $$

Or $p^*$ is the equilibrium coefficient for vaporization. Now since K is characteristic of a reaction at a given temperature, this would imply that if we change x, then the new partial pressure at equilibrium would change according to